Semiconducting polymers and low molecular weight organic molecules have received significant attention for their application as active layers in organic solar cells, due to their potential low cost, high mechanical flexibility, wide array of functionalities, and well-understood structure-composition-property relationships. In fact, both solution-printed polymer solar cells and vacuum-deposited small molecule organic solar cells have independently reached record certified efficiencies of around 10%, through careful materials selection and device architecture engineering.
Vapor-deposited polymer solar cells would enhance the ability to integrate attractive materials into organic solar cells. For example, unsubstituted polymers, which are reported to be more stable because their highly compact structures prevent oxygen permeation into the polymer bulk, are also insoluble and infusible because of their compact structures. Furthermore, the vacuum fabrication of multilayered devices is not constrained by the requirement of finding solvents that will not dissolve the underlying layers to prevent mixing between layers during deposition. However, the high temperatures necessary to physically deposit polymers by vacuum thermal evaporation leads to polymer degradation, limiting materials to low-molecular-weight organics. The few reports of the use of a vapor deposition technique (such as physical deposition, plasma polymerization, and thermal chemical vapor deposition) to deposit a polymer photoactive layer resulted in low corresponding device efficiencies (<0.3%). Thus a soluble derivative (e.g. poly(3-hexylthiophene)) or an oligomeric version are typically used to facilitate processing by standard solution printing or vacuum thermal evaporation.